黄河下游泥沙沉积汇对入海悬移质泥沙粒度的影响

黄河下游泥沙沉积汇在黄河流域系统的泥沙收支平衡(Sediment budget)中起着重要的作用。通过河床主槽中泥沙的淤积和冲刷,运动泥沙的组成发生变化;通过滩地上泥沙淤积和河岸坍塌,河漫滩上前期淤积的泥沙与洪水所挟带的泥沙发生交换,使悬移质泥沙的组成发生变化。泥沙冲淤量对不同粒径组入海泥沙百分比的影响是不同的。小于0.01mm细泥沙的百分比与下游河道淤积量呈正相关,0.025~0.05mm和大于0.05mm的较粗泥沙的百分比与下游河道淤积量呈负相关。入海泥沙平均粒径与深泓年摆幅之间也存在着负相关关系。自20世纪60年代,中期以来,特别是自80年代中期以来,由于黄河下游径流量显著减小,黄河下游河道发生萎缩,河宽减小,因而主流线的摆动幅度也显著减小。这使得原来十分强烈的河岸侵蚀和滩槽泥沙交换强度大为减弱,主槽洪水得到的来自河岸侵蚀的细粒泥沙越来越少,因而使入海泥沙粒径变粗。

Effect of sediment sink of lower Yellow River on grain size of sediment flux to the sea

The lower Yellow River can be regarded as one of the sediment sinks in the river drainage basin,which plays an important role in the sediment budget of the river system.Through sediment deposition and scour in main channel,composition of sediment in motion changes;through sediment deposition on floodplain and bank erosion,exchange occurs between the sediment on floodplain and the sediment in motion.Thus,composition of suspended sediment changes along the river.It is found that sediment deposition has different effect on various fractions in sediment flux to the sea.The percentage of fractions finer than 0.01mm in the sediment flux to the sea is positively correlated with sedimentation amount in the lower Yellow River,but negative correlations exist between sedimentation amount and the percentages of fractions in the range from 0.025mm to 0.05 mm and coarser than 0.05mm in the sediment flux to the sea.The average diameter of sediment flux to the sea is found to be negatively correlated with the annual shift of the thalweg of the lower Yellow River.Since the mid-1960s,especially since the 1980s,water discharges of the lower Yellow River decreased significantly and the channel shrinkage occurred,the channel width decreased and so did the annual shift of the river's thalweg.As a result,bank erosion weakened,and the sediment exchange between main channel and floodplain became less intensive than that in the previous period.Since the floods in the main channel could get less fine sediment from the floodplain,the grain size of suspended sediment flux to the sea tended to become coarser.